Double metal cyanide (DMC) complexes are well-known catalysts for epoxide polymerization. These catalysts are highly active and give polyether polyols having low unsaturation, a very narrow molecular weight distribution and consequently, a low polydispersity.
DMC catalysts were discovered more than forty years ago by researches from the General Tire and Rubber Company (U.S. Pat. Nos. 3,404,109; 3,427,256; 3,427,334 and 3,941,849). Since then, the technology for producing DMC catalysts has been improved by different companies.
Conventional process for the preparation of DMC catalysts directed to the epoxide polymerization is well-disclosed in EP0090444, EP0090445, EP1022300, EP0555053, EP0700949, EP0894108 and EP0932445. This process involves the reaction of aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. A low molecular weight organic complexing agent, typically an ether or an alcohol such as tert-butyl alcohol, is included in the catalyst preparation. The activity of DMC catalysts has been greatly enhanced by incorporating, in addition to the organic complexing agent, functionalized ligands such as polyether polyols.
The resulting water-insoluble double metal cyanide complex catalyst which precipitates from solution must thereafter be recovered from the aqueous reaction medium, washed to remove undesirable by products and impurities, and dried in order to obtain the catalyst in a form suitable for use in a polymerization process.
To this aim, the solid complex is firstly reslurried in a mixture of an organic complexing agent, generally tert-butyl alcohol, and water and thereafter in pure organic complexing agent, being subsequently filtered and dried under vacuum at a moderate temperature.
Different modifications with respect to the order of addition of reactants or the time to incorporate the organic complexing agent have been published. For example, EP0555053 describes the addition of the aqueous solution of the metal salt over the aqueous solution of the metal cyanide salt, whereas in EP0743093 the reverse order of addition is disclosed, showing the advantageous effect on the activity of the resulting DMC catalyst.
However, there are few documents describing the effect of the washing step on the catalytic activity. EP0700949 shows that the washing step is necessary in order to give an active catalyst and that multiple washing steps could lead to the preparation of even more active catalysts. Said step is carried out by washing the precipitate with an aqueous solution of tert-butyl alcohol (70%).
Some references mention that an excessive use of water in the washing step should be avoided, since the excess of the non-reacting metal salt could be removed giving rise to a lower-active catalyst. In addition, in order to suit the drying of the catalyst, is desirable to wash it with the organic complexing agent only or with a mixture of water and organic complexing agent (EP0555053). In EP0894108, the catalyst is washed with an aqueous solution containing the organic complexing agent and a polyether polyol in a range 40-70% and 0.1-8%, respectively.
The isolation of the dry, active double metal cyanide complex catalyst is generally complicated and therefore, a convenient and effective method of preparing said catalysts which can be readily isolated by conventional and straight-forward filtration techniques would be of particular interest.
On the other hand, it has been shown (EP0700949, EP0894108, EP0932445) that the incorporation of polyether polyols or functionalized polymers to the catalyst, in addition to the metal salt, the metal cyanide salt and the organic complexing agent, provides improved DMC catalysts with higher activities and allows the production of polyether polyols with low catalyst concentrations.
In EP0090444, the suspension of a DMC catalyst in a propoxilated glycerol with molecular weight ranged from 200 to 400 is described. In EP0700949 it is shown that the use of polyether polyol ligands with molecular weights higher than 500 provides DMC catalysts with an improved activity. EP0894108 discloses catalysts capable of polymerizing propylene oxide at a rate in excess of 1 Kg PO/g Co/min at 100 ppm catalyst, based on the weight of finished polyether, at 105° C., said catalyst having polyethers with a number average molecular weight less than 500, and no tertiary hydroxyl groups. However, all of the examples of these documents describe the use of polyether polyols obtained by basic catalysis and the state of the art is silent about the influence of the acidic nature of the polyether polyol ligands.
DMC catalysts with improved properties are thus needed.
In spite of the different procedures to provide DMC catalysts having good activity for epoxide polymerization, catalyst with even improved activity are still desirable in order to reduce the catalyst level used in said polymerization reactions.
Furthermore, since DMC catalysts generally require activation times higher than one hour which have a negatively impact on the polymerization cycle times, it is also desirable to reduce said activation times.